On Fri, May 25, 2018 at 08:19:00AM +0200, Christoph Hellwig wrote: > On Thu, May 24, 2018 at 02:13:56PM -0400, Brian Foster wrote: > > Ok, so I guess writeback can see uptodate blocks over a hole if some > > other block in that page is dirty. > > Yes. > > > Perhaps we could make sure that a > > dirty page has at least one block that maps to an actual extent or > > otherwise the page has been truncated..? > > We have the following comment near the end of xfs_writepage_map: > That comment is what I'm basing on... > /* > * We can end up here with no error and nothing to write if we > * race with a partial page truncate on a sub-page block sized > * filesystem. In that case we need to mark the page clean. > */ > So we can correctly end up with nothing to write on a dirty page, but it presumes a race with truncate. So suppose we end up with a dirty page, at least one uptodate block, count is zero (i.e., due to holes) and i_size is beyond the page. Would that not be completely bogus? If bogus, I think that would at least detect the dumb example I posted earlier. Brian > And I'm pretty sure I managed to hit that case easily in xfstests, > as my initial handling of it was wrong. So I don't think we can > even check for that. > > > I guess having another dirty block bitmap similar to > > iomap_page->uptodate could be required to tell for sure whether a > > particular block should definitely have a block on-disk or not. It may > > not be worth doing that just for additional error checks, but I still > > have to look into the last few patches to grok all the iomap_page stuff. > > I don't think it's worth it. The sub-page dirty tracking has been one > of the issues with the buffer head code that caused a lot of problems, > and that we want to get rid of. > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html
